Oxygen is currently widely used as a therapeutic agent in patients who have, or who are presumed to have, tissue hypoxia. Though any tissue can potentially be injured by oxygen, the lung appears to be particularly susceptible to injury from exposure to molecular oxygen at high concentrations. It is as yet uncertain what might be the maximum safe concentration and/or duration of oxygen that would avoid substantial tissue injury. Clinically, it is generally assumed that prolonged administration of greater than 60% oxygen is hazardous, that less than 40% oxygen is acceptable, and that 40- 60% oxygen is of unknown risk. However, this clinical impression is imprecise because it has not been unsubstantiated scientifically. There is currently no direct, noninvasive clinical diagnostic test to identify oxygen toxicity in its early stages; thus it is usually recognized only after significant functional and morphological injury has already occurred. Our current understanding of the illness is so limited that the only two general therapeutic avenues open are the restriction of oxygen administration and general supportive management of the critically ill patient. Prognosis is guarded, particularly when the patient has the closely related and devastating entity of adult respiratory distress syndrome (ARDS). This pilot project represents the application of highly sensitive new techniques to determine whether therapeutic oxygen supplementation results in the elaboration of identifiable chemical mediators which have the potential for either inducing the tissue injury associated with oxygen toxicity or serving as markers which quantify the risk of such injury, or both.